You get the all the killer enhancing earrings from garrison. They don't increase damage though, they make it so you intimidate enemies. They need to be attacking you though so it's mainly for tanks and people playing solo.

This earring is mostly for high level tanks in exp parties. Once you start exping off weapons in Ro'Maeve, Ru'Aun and Ve'Lugannon palace, this is extremely useful because "enhances" actually grants the Arcana Killer trait. When a monster is targeting the tank and he has the appropriate killer trait, it occasionally causes the monster to be paralyzed with a "Monster is intimidated by (tank)'s presence" message. This earring is probably the most useful for high level EXP out of all these types of earrings. And just so you know, Arcana and Undead are weak vs. each other, just like light vs. dark and demons vs. dragons.

I believe the idea is that the earring makes you more like an undead. Thus you are less likely to kill your own kind, and more likely to deal massive damage to the mob class opposite you as an undead (in this case, arcana).

I could see a dark knight farming arcana for some reason making use of this earring. But really its just a novelty, imo.